The present invention relates to new and improved powder pressed materials for thermoelectric applications and a method for making the same.
It has been recognized that the world supply of fossil fuels for the production of energy is being exhausted at ever increasing rates. This realization has resulted in an energy crisis which impacts not only the world's economy, but threatens the peace and stability of the world. The solution to the energy crisis lies in the development of new fuels and more efficient techniques to utilize them. To that end, the present invention deals with energy conservation, power generation, pollution, and the generation of new business opportunities by the development of new materials for use in devices which provide more electricity.
An important part of the solution with respect to the development of permanent, economical energy conversion lies in the field of thermoelectrics wherein electrical power is generated by heat. It has been estimated that two-thirds of all our energy, for example, from automobile exhausts or power plants, is wasted and given off to the environment. Up until now, there has been no serious climatic effect from this thermal pollution. However, it has been predicted that as the world's energy consumption increases, the effects of thermal pollution will ultimately lead to a partial melting of the polar ice caps with an attendant increase in sea level. Employment of the waste heat for the regeneration of electricity can provide a direct reduction in the thermal pollution, independent of the source of the energy.
The performance of a thermoelectric device can be expressed in terms of a figure of merit (Z) for the material forming the device, wherein Z is defined as: ##EQU1## where:
Z is expressed in units x 10.sup.3
S is the Seebeck coefficient in .mu.V/.degree.C.
K is the thermal conductivity in mW/cm-.degree.K.
.sigma. is the electrical conductivity in (.OMEGA.-cm).sup.-1
From the above, one can see that in order for a material to be suitable for thermoelectric power conversion, it must have a large value for the thermoelectric power Seebeck coefficient (S), a high electrical conductivity (.sigma.), and a low thermal conductivity (K). Further, there are two components to the thermal conductivity (K): K.sub.l, the lattice component; and K.sub.e, the electrical component. In non-metals, K.sub.l dominates and it is this component which mainly determines the value of K.
Stated in another way, in order for a material to be efficient for thermoelectric power conversion, it is important to allow carriers to diffuse easily from the hot junction to the cold junction while maintaining the temperature gradient. Hence, high electrical conductivity is required along with low thermal conductivity.
Thermoelectric power conversion has not found wide usage in the past. The major reason for this is that prior art thermoelectric materials which are at all suitable for commercial applications have been crystalline in structure. Those crystalline materials best suited for thermoelectric devices are very difficult to manufacture because of poor mechanical properties and sensitivity of material properties to compositional changes. This results because they contain a predominance of elements, such as tellurium, and selenium which are natural glass formers. The growth, control, and mechanical stability of these crystals have, therefore, led to what to this date are insurmountable problems. In particular, the materials having a high figure of merit (Z) are generally made from chalcogenides such as tellurium compounds which are notorious for the difficulties in growth of high quality single crystals. Even when such crystals are grown, they contain large densities of defects and are often unstable. In addition, they usually are far from stoichiometric. For all of these reasons, controlled doping has proven to be extremely difficult.
Crystalline solids cannot attain large values of electrical conductivity while maintaining low thermal conductivity. Most importantly, because of crystalline symmetry, thermal conductivity cannot be controlled by modification.
In the case of the conventional polycrystalline approach, the problems of single crystalline materials still dominate. However, new problems are also encountered by virtue of the polycrystalline grain boundaries which cause these materials to have relatively low electrical conductivities. In addition, the fabrication of these materials is also difficult to control as a result of their more complex crystalline structure. The chemical modification or doping of these materials, because of the above problems are especially difficult.
Among the best known currently existing polycrystalline thermoelectric materials are (Bi, Sb).sub.2 Te.sub.3, PbTe, and Si-Ge. The (Bi, Sb).sub.2 Te.sub.3 represents a continuous solid solution system in which the relative amounts of Bi and Sb are 0 to 100%. The Si-Ge materials are best suited for high temperature applications in the 600.degree. to 1000.degree. C. range with a satisfactory Z appearing at above 700.degree. C. The PbTe polycrystalline material exhibits its best figure of merit in the 300.degree. to 500.degree. C. range. None of these materials is well suited for applications in the 100.degree. C. to 300.degree. C. range. This is indeed unfortunate, because it is in this temperature range where a wide variety of waste heat applications are found. Among such applications are geothermal waste heat and waste heat from internal combustion or diesel engines in, for example, trucks, buses, and automobiles. Applications of this kind are important because the heat is truly waste heat. Heat in the higher temperature ranges must be intentionally generated with other fuels and therefore is not truly waste heat.
Improved thermoelectric materials have been developed which are not single phase crystalline materials, but instead, are disordered materials. These materials, fully disclosed in U.S. application Ser. No. 341,864, filed Jan. 22, 1982 in the names of Tumkur S. Jayadev and On Van Nguyen for New Multiphase Thermoelectric Alloys and Method of Making Same, now abandoned in favor of U.S. Ser. No. 412,306 filed Aug. 27, 1982 and to issue May 8, 1984 as U.S. Pat. No. 4,447,277, and incorporated herein by reference, are multiphase materials having both amorphous and multiple crystalline phases. Materials of this type are good thermal insulators. They include grain boundaries of various transitional phases varying in composition from the composition of matrix crystallites to the composition of the various phases in the grain boundary regions. The grain boundaries are highly disordered with the transitional phases including phases of high thermal resistivity to provide high resistance to thermal conduction. Contrary to conventional materials these materials have grain boundaries defining regions including conductive phases therein providing numerous electrical conduction paths through the bulk material for increasing electrical conductivity without substantially effecting the thermal conductivity. In essence, the materials have all of the advantages of polycrystalline materials in desirably low thermal conductivities and crystalline bulk Seebeck properties. However, unlike the prior art polycrystalline materials, these disordered multiphase materials also have desirably high electrical conductivities. Hence, as disclosed in the aforementioned referenced application, the S.sup.2 .sigma. product for the figure of merit of these materials can be independently maximized with desirably low thermal conductivities for thermoelectric power generation.
Amorphous materials, representing the highest degree of disorder, have been made for thermoelectric applications. The materials and methods for making the same are fully disclosed and claimed, for example, in U.S. Pat. Nos. 4,177,473, 4,177,474 and 4,178,415 which issued in the name of Stanford R. Ovshinsky. The materials disclosed in these patents are formed in a solid amorphous host matrix having structural configurations which have local rather than longrange order and electronic configurations which have an energy gap and an electrical activation energy. Added to the amorphous host matrix is a modifier material having orbitals which interact with the amorphous host matrix as well as themselves to form electronic states in the energy gap. This interaction substantially modifies the electronic configurations of the amorphous host matrix to substantially reduce the activation energy and hence, increase substantially the electrical conductivity of the material. The resulting electrical conductivity can be controlled by the amount of modifier material added to the host matrix. The amorphous host matrix is normally of intrinsic-like conduction and the modified material changes the same to extrinsic-like conduction.
As also disclosed therein, the amorphous host matrix can have lone-pairs having orbitals wherein the orbitals of the modifier material interact therewith to form the new electronic states in the energy gap. In another form, the host matrix can have primarily tetrahedral bonding wherein the modifier material is added primarily in a non-substitutional manner with its orbitals interacting with the host matrix. Both d and f band materials as well as boron and carbon, which add multiorbital possibilities can be used as modifiers to form the new electronic states in the energy gap.
As a result of the foregoing, these amorphous thermoelectric materials have substantially increased electrical conductivity. However, because they remain amorphous after modification, they retain their low thermal conductivities making them well suited for thermoelectric applications, especially in high temperature ranges above 400.degree. C.
These materials are modified on an atomic or microscopic level with the atomic configurations thereof substantially changed to provide the heretofor mentioned independently increased electrical conductivities. In contrast, the materials of U.S. Pat. No. 4,447,277 are not atomically modified. Rather, they are fabricated in a manner which introduces disorder into the material on a macroscopic level. This disorder allows various phases including conductive phases to be introduced into the materials much in the same manner as modification atomically in pure amorphous phase materials to provide controlled high electrical conductivity while the disorder in the other phases provides low thermal conductivity. These materials therefore are intermediate in terms of their thermal conductivity between amorphous and regular polycrystalline materials.
The materials of the present invention can be produced from materials by methods embodying the present invention which have the same structural characteristics as the materials disclosed and claimed in the aforementioned and referenced U.S. Pat. No. 4,447,277. The materials of the present invention however include a form of bulk disorder not present in the materials disclosed in the aforementioned application. This additional disorder results from the fact that the materials of the present invention are in the form of compacted or pressed powder material. While the individual powder particles from which the compacted material is formed can have the desirable disordered structure as previously described, the additional disorder resides in the interfaces between the compacted particles. Although this additional disorder results in lower electrical conductivity, the thermal conductivity is also lowered to an extent to provide a figure of merit which is acceptable for commercial applications, especially in the temperature range up to about 400.degree. C. In fact, the figure of merit of these materials is greater than the figure of merit of the conventional prior art polycrystalline materials above about 200.degree. C.
In addition to the foregoing, the materials of the present invention can be compacted or pressed into solid form with dimensions suitable for direct application as thermoelectric elements for thermoelectric devices. Also, the elements can be formed at commercially acceptable high rates to sustain high volume mass production of thermoelectric devices. This is not possible with conventional polycrystalline materials which must be grown over long periods of time and under precisely controlled growing conditions.